Support comparing libraries/packages for type compatibility

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="96" height="96"hspace="10"> Issue by munificent
Originally opened as dart-lang/sdk#2828

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One planned feature of pub is the ability to have it select an appropriate configuration-specific package based on the environment the app is running in. For example, there may be a single "unittest" package that has implementations for running on the standalone VM (just printing to stdout) and the browser (generating HTML).

For this to make sense, every implementation must expose the same (or a compatible) API. Since libraries aren't first class, the current plan is to basically duck type it: it's up to the package designer to ensure all of the implementations match.

It would be very handy if pub had a feature that would let you compare two libraries/packages and would determine if their public APIs are compatible with each other. This could also be useful for upgrading a package so you can see which API changes are breaking.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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Added this to the Later milestone.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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Something along these lines would be useful, but is outside of the scope of pub.

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Added NotPlanned label.